The present invention relates generally to a projection optical system, and more particularly to a projection optical system with a high numerical aperture (“NA”) used to expose an object, such as a single crystal substrate for a semiconductor wafer and a glass plate for a liquid crystal display (“LCD”).
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project and transfer a circuit pattern on a reticle (or a mask) as a first surface-onto a wafer, etc. as a second surface, to manufacture such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology. The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus, which is also called a resolution, is proportional to a wavelength of light used for exposure, and inversely proportional to the NA of the projection optical system. The resolution quality is better at shorter wavelengths.
Decreasing the exposure light wavelength and increasing the NA of the projection optical system have been promoted to meet recent demands for finer semiconductor devices. An early exposure apparatus began with a development of a g-line stepper that uses a g-line ultra-high pressure mercury lamp (having a wavelength of about 436 nm) as a light source and includes a projection optical system with a NA of about 0.3, then an i-line stepper that uses an i-line ultra-high pressure mercury lamp (having a wavelength of about 365 nm) as an light source, and a stepper that uses a KrF excimer laser (having a wavelength of about 248 nm) and includes a projection optical system with a NA of about 0.65. Current widespread projection exposure apparatuses replace these steppers with scanners that use a KrF excimer laser and ArF excimer laser (with a wavelength of approximately 193 nm) as a light source and can include a high-NA projection optical system. The currently commercially available projection optical system with the highest NA has a NA=0.8. The stepper is a step-and-repeat exposure apparatus that moves a wafer stepwise to an exposure area for the next shot for every shot of the cell projection onto the wafer. The scanner is a step-and-scan exposure apparatus that exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask, and by moving, after the exposure shot, the wafer stepwise to the next exposure area to be shot.
Scanners that use F2 laser (with a wavelength of approximately 157 nm) as a light source as well as the KrF and ArF excimer lasers and include a projection optical system with NA=0.85 have been extensively studied. There is demand for the development of a projection optical system with NA of 0.90.
With such a development of the projection optical system, an antireflection coating has been developed for applications of an optical element in the projection optical system. The applied antireflection coating technology for the visual light used for conventional cameras, etc. can develop an antireflection coating without any significant problem to an exposure apparatus that uses a g-line ultra-high pressure mercury lamp as a light source. A design of an antireflection coating for the exposure apparatus that uses the g-line ultra-high pressure mercury lamp is almost applicable to an exposure apparatus that uses an i-line ultra-high pressure mercury lamp although some coating materials are unsuitable for the antireflection coating due to optical absorptions.
No high index materials (with a refractive index of about 2.0 or higher) are proposed which have excellent transmission characteristics for light having a wavelength of 300 nm or smaller (or an excellent transmittance) for exposure apparatuses that uses KrF and ArF excimer lasers as a light source. Therefore, antireflection coating materials are limited to low index materials (with a refractive index of 1.4 to 1.45), such as SiO2 and MgF2, and middle index materials (with a refractive index of about 1.6), such as Al2O3 and LaF3. It is very difficult to develop the antireflection coating due to problems of a design of the coating and control over the coating thickness.
The improved multilayer design technology for the antireflection coating associated with computer developments has solved the coating design problem, and provided conventionally acceptable design values to antireflection coatings made of the low and middle index materials. Since this configuration replaces a coating layer that to be originally made from a high index material, with a coating layer made of the low and middle index materials to form an equivalent coating layer, the designed coating thickness is smaller than that of the antireflection coating in the exposure apparatuses that use g-line and i-line as a light source.
One solution for the problem of control over the coating thickness is the newly proposed coating thickness monitoring using a quarts resonator in addition to the conventionally used optical monitoring of the coating thickness. The sputtering coating formation has been established which is superior to the vacuum evaporation coating formation in controlling coating thickness and coating quality uniformity.
Currently, an antireflection coating having excellent reflective characteristics of up to NA=0.8 can be designed and manufactured (see, for example, Japanese Patent Applications, Publication Nos. 2001-4803 and 2000-357654).
However, an exposure apparatus that uses a F2 laser as a light source cannot use an oxide as a coating material since oxygen strongly absorbs the light having a wavelength near 157 nm, and has the limited coating materials, for example, low index materials (with a refractive index of about 1.4 to 1.55), such as MgF2 and AlF3, and middle index materials (with a refractive index of about 1.70 to 1.75), complicating the design and manufacture of the antireflection coating.
In addition, a transmission loss increases or the transmittance lowers due to optical absorptions by the antireflection coating in a wave range of the ArF excimer laser and F2 laser. A conventionally negligible transmission loss in a coating layer becomes problematic due to light scattering in a wave range of the F2 laser, lowering the throughput and deteriorating the resolution.
As discussed, a design and manufacture of the antireflection coating pose no problem in developing a projection optical system with NA=0.8, whereas it becomes difficult to design and manufacture the antireflection coating having excellent reflective characteristics when the NA becomes 0.85 or higher. As a result, it is unable to provide a projection optical system that has both an NA of 0.85 or greater and excellent optical performance.